1. Field of The Invention
The present invention relates to an apparatus for monitoring the toxicant contamination of water by using aquatic animals, more particularly to an apparatus for detecting the contamination of water with toxic material by monitoring the behavior of the aquatic animals bred in the water to be tested by means of the image processing technique.
2. Description of The Related Art
In a water purification plant, it must be always monitored whether or not toxic material, especially acute toxicity, is mixed in raw water. If the toxicant contamination of raw water is detected, the intake of raw water into the plant has to be stopped immediately. As a matter of course, it is necessary to monitor that toxic material is not mixed in the purified water which is processed in the plant. Also in a sewage treatment plant, it must be monitored whether or not toxic material is mixed in the water which is processed in the plant, before the processed water is discharged therefrom.
In this manner, it is necessary in every water processing plant to always monitor whether or not toxic material is mixed in the water flowing into the plant and/or flowing out therefrom. To this end, aquatic animals such as carp, crucian carp, dace, bitterling, rainbow trout and so on have been bred in a basin supplied with a part of water to be tested, and a person has to watch, by the visual observation, the abnormal behavior of such aquatic animals occurring when acute toxicity is mixed in the water. Therefore, it has been difficult to continuously watch the behavior of the aquatic animals and to detect early the toxicant contamination of the water to be tested.
To improve this, there has been proposed an apparatus as disclosed in the article "System for sensing toxicant in raw water" in pages 464 to 466 of the proceedings of The 36th Annual Conference of Japan Waterworks Associaton held in Miyazaki, Japan, in May, 1985. According to the method disclosed therein, a fish selected from among those as mentioned above is bred in a basin, which is supplied with a part of water to be tested. In the article, the water to be tested is considered as being contaminated with acute toxicity, when the fish drifts on the surface of the water often or for long time, turning its venter upward.
The basin is lighted from above and the behavior of the fish is observed by a television camera installed above the basin, i.e., on the same side as a light source. In an image taken by the camera, the fish drifting in such a manner as mentioned above is recognized as a bright spot which is independent and larger than a predetermined size. The toxicant contamination of water is judged on the basis of the existence of the bright spot in the image and/or the time-dependent change in the occurrence rate thereof.
However, the prior art as mentioned above has some problems. When light is projected to a fish drifting on the surface of water from above, there is the difference in the brightness of the light reflected from the fish in accordance with parts thereof. For example, the light reflected by the ventral part of the fish is brightest and that reflected by the dorsal part thereof is darkest. The brightness of light reflected by various fins of the fish lies between those of lights reflected by the ventral and dorsal parts thereof.
In order that a fish is clearly recognized, distinguished from a background, there must be a considerable difference in the brightness between the fish and the background. Namely, the brightness of the background is necessary to be selected at the value much brighter than that of the ventral part the fish or at the value much darker than that of the dorsal part thereof. However, the color of the dorsal and ventral parts of a fish changes through a year and the intensity of light reflected therefrom also changes accordingly. Therefore, it is difficult and troublesome to determine and change the level of the brightness of the background in accordance with the seasonal change in the color of a body of a fish.
Further, the brightness of the reflected light also changes in accordance with the postural angle of a fish with respect to the light projected thereon and therefore the ventral part of the fish often shines in silver when the fish turns. The inventors have found through their experiment that, from such various factors as mentioned above, it is very difficult to recognize a fish at high accuracy through a year by means of the image recognition technique according to the prior art, even though the brightness of the background is selected to be deep black or pure white. If water to be tested becomes turbid, the accuracy of recognition is further deteriorated.